Methods and materials for the removal of the sensitizing dye(s) from electrophotograpic prints



United States Patent METHODS AND MATERIALS FOR THE REMOVAL OF THE SENSITIZING DYE(S) FROM ELECTRO- PHOTOGRAPHIC PRINTS Donald R. Eastman, Rochester, N.Y., assignor to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey No Drawing. Filed Oct. 18, 1961, Ser. No. 145,983

. 8 Claims. (Cl. 96-1) This invention is related to methods and materials for the removal of sensitizing dyes from electrophotographic prints made on zinc oxide resinous layers and in particular Where this decolorization is accomplished during a separate processing step after image formation and stabilization.

Electrophotographic methods for reproducing images are well known. In one of these processes known as xerography, a photoconductive layer coated on a conductive support is given a uniform electric charge and exposed to a pattern of activating radiation which creates a corresponding electrostatic latent image on the surface of a photoconductive layer by dissipating the charge of the layer in the areas receiving the activating radiation in accordance with the amount of activating radiation. This electrostatic latent image is then converted into a visible image by depositing on it an opaque electroscopic material by electrostatic attraction. This image is then fixed by any of the well known methods such as application of heat and/ or pressure. In another of these processes known as photoconductography, a photoconductive layer coated on the conductive support is given an image exposure. The photo-conducting material is nonconducting before exposure and becomes electrically conducting upon and after exposure. The exposed element is then developed by the application of an electrolytic developer solution to the photoconductive surface and the passage of an electric current through the conducting areas of the photoconductive layer and the electrolytic developer solution to produce a visible image.

Such photoconductive layers are inherently sensitive to light in the ultraviolet range of the spectrum; and, in ordl'gtO extend the light sensitivity into the visible region, it is common practice to sensitize the zinc oxide with optical sensitizing dyes such as Rose Bengal, Fluorescein, Crystal Violet, etc. For some purposes, the color of the sensitizing dye is undesirable in the processed print. It is known to use concentrated aqueous solutions of acids such as hydrochloric acid, sulfuric acid, etc., or alkali hydroxides to dissolve and remove the colored zinc oxide from the photoconductive surface following image development. This process actually removes the zinc oxide, leaving the photoconductive coating colorless and transparent except for the image on the surface of the coating. Removal of the Zinc oxide is undesirable for some applications where the processed element is to be used as a print rather than as a transparency.

It is, therefore, an object of my invention to provide a novel process for removing sensitizing dyes from exposed and image developed eleotrophotographic prints. A further object is to provide a method for desorbing and/or decolorizing sensitizing dyes which does not remove the zinc oxide from the photoconductive layer or damage the image material. Another object of this invention is to decolorize layers in the nonimage areas so that they will be as white or whiter than similar undyed layice ers when coated on the same substrate. Another object of this invention is to provide a decolorization step rapid enough to be applicable to mechanization. Still other objects will become apparent from the following specification and claims.

According to my invention, the image bearing xerographic or photoconductographic print is treated with an organic solvent or solvent mixture for the polymeric binder either alone or in conjunction with certain decolorizing agents. By solvent, I mean an organic liquid which will dissolve (given enough time), soften or penetrate the polymeric binder material in the photoconductive layer. It a single solvent system is impractical because it attacks and damages the layers before decolorization is completed, the primary solvent is diluted with a miscible nonsolvent or with a milder solvent for the particular polymeric binder used. One particular feature of thisinvention is that the solvent, solvent mixture or solvent-nonsolvent mixture is always applied under conditions such that the zinc oxide binder layer is not damaged by the application. That is, if a strong solvent is used, it is applied for only a short time, for instance, by spraying, while if a weaker solvent is applied, it may be left in contact with the layer for a longer time.

Depending on.:the particular binder polymer(s), sensitizing dye(s), binder solvent(s) involved and the chemi cal stability of the image material, the sensitizing dye(s) may be effectively removed by one of the following three modes:

(1) Immersing the print in a suitable solvent, solvent mixture, or solvent-nonsolvent mixture for the binder polymer which desorbs the dye(s), or by (2) Immersing the print in a solvent, solvent mixture, or solvent-nonsolvent mixture for the binder polymer in which is dissolved a decolorization agent(s) which can either (A) accelerate the desorption of the dye(s) and/ or (B) render the dye(s) colorless, or by (3) Applying to the print surface or immersing the print in a binder solvent in which is dissolved an oxidizing agent which oxidizes the dye(s) to a colorless form.

The spectrally sensitized electrophotographic element used in my process may be any of those which have been described in the prior art. The electrophotogr-aphic element consists of (1) a conductive material, such as a metal plate, that is, brass, aluminum, zinc, etc., a nonconductive material, such as paper, glass, wood, etc., which has been made conductive by a thin coating of evaporated metal or laminated metal foil, etc., or other material such as tin oxide, carbon, etc., over which is coated (2) a photoconductive layer. The photoconductive layer usually comprises a dispersion of zinc oxide in aresinous binder, such as a polyvinyl resin, e.g., polystyrene, polyvinylacetate, polyvinyl chloride, etc., and copolymers of these, the silicone resins, cellulose esters and cellulose ethers, and the acrylic resins, e.-g., the poly(methacrylates), the poly(methyl methacrylates), etc. These resins are hydrophobic and must have good electrical insulating properties. Various optical sensitizers are used to extend the optical sensitivity of the zinc oxide. These include triphenylmethane dyes, such as Rose Bengal, Fluorescein, Crystal Violet, etc., ketoimine dyes, such as Auramine B, etc., xanthene dyes, such as Rhodamine B, etc.

Included among the organic solvents used advantageously according to my invention are the hydrocarbons, such as benzene, toluene, xylene, heptane, hexane, etc.,

the halogenated hydrocarbons, such as methylchloroform, trichlorofluoromethane, tetrachloromethane, 1,1,2-trichloro 1,2,2 .trifiuor oethane (Freon 113 made by Du Pont), 1,2-dichloroethane, etc., the amides, such as dimethylformamide, etc., the ketones, such as acetone, methyl ethyl ketone, methyl isobutyl 'ketone, etc., p-dioxane, tetrahydrofuran, etc. These solvents are used individually or in mixtures depending upon the particular hinder or binders and sensitizing dye or dyes used in the photoconducting layer that is to be decolorized. Alcohols, such as methyl alcohol, ethyl alcohol, etc., are solvents for the methacrylate binders but are nonsolvents for other polymer systems. In certain instances, it is desirable to add a misci'ble nonsolvent to the solvent or solvent mixture as a diluent or-as a solvent for a decolorizing agent when the decolorizing agent is not soluble enough in the binder solvent.

Decolorization agents that are used advantageously I dissolved in one of my solvents or solvent mixtures to accelerate desorption of the senstitizing dye(s) and/or to render the dye(s) colorless include the monobasic unsaturated acids, such as acrylic acid, crotonic acid, etc., the dibasic acids, such as maleic acid, fumaric acid, etc., saturated monobasic acids, such as acetic acid, chloroacetic acid, the saturated dibasic acids, such as oxalic acid, glutaric acid, etc., aromatic acids, such as benz-oic acid, ,d-naphthoic acid, etc.

Various oxidizing agents are used to advantage in the solvent solution to decolorize those dyes which are convertible to a colorless leuco form. 1,3-dichl-oro-5,5-dimethylhydantoin is particularly efiicacious as an oxidizing agent for this purpose.

The method used to apply the treatment solutions can be varied considerably and will depend upon the character-istics of the electrophotographic material as Well as the solvents .used for the treatment. The prints may be immersed flatin a tray, suspended in the vertical position in a tank of the treatment solution or passed through the treatment tank in a continuous manner. Alternatively, the treatment bath may be sprayed on or rolled onto the surface of the photoconductive layer. After treatment, the electrophotographic element may be, in some cases, blotted or squeegeed and then dried in air or by forced drying. Most of the solvents used are quite volatile and drying occurs rapidly.

The following examples will serve to illustrate the three methods for desorbin-g or decolorizing the sensitizing dyes in electrophotographic elements.

EXAMPLE 1 A sheet of photoconductive material consisting of a layer of zinc oxide sensitized with the triphenylmethane dye, Crystal Violet, in a binder polymer system comprising Pliolite 8-7 (a solution of a synthetic styrene copolymerresin in toluene made by Goodyear Tire and Rubber Company), Silicone SR'82 resin (made by General ElectIic Company) and Piccopale resin (a petroleum hydrocarbon product made by Pennsylvania Industrial Chemical Corporation) coated on aluminum foil-paper laminate support was bathed with agitation for 30 seconds in 100 ccsfof a solvent mixture composed of four parts methylchloroform, two parts dimethylformamide and ..1 part water (all by volume). The dye was completely desorbed from the coating by this treatment. The solvent mixture was colored by the .dyes but the air-dried treated layer was whiter than a similar undyed and untreate zinc oxide polymer layer.

The next three examples illustrate sensitized photoconductive layers from which thesensitizing dyes are not desorbed or decolorized by treating them with a solvent or solvent mixture but which are completely decolorized by treatment with a solvent solution of a desorption accelerator or an oxidizing agent.

4 EXAMPLE 2 Table 1 Change produced in photo- Composition of treatment bath 7 conducting layer Mehtylchloroform, 4 cc... Diemthyliormamide, 2 cc Water, .1 cc

Acetone Acetone, cm Maleic acid, 1 cc .1

Incomplete decolorization.

No decolorization.

Complete desorption of dyes and decolorization.

EXAMPLE 3 A xerographic print was made on a charged sheet of photoconduetive material consisting of a layer of zinc oxide sensitized with Auramine Base and Rhodamine B in a methacryl-ate resin binder (Lucite 46 made by Du Pont) coated on a pare support, by depositing carbon pigmented toner material (resinous) from a magnetic brush onto the layer surface which had an image-wise charge pattern induced by exposure to actinic radiation. The toner was fixed by fusion at approximately 300 F.

Treatment of the print for 30 seconds with agitation in ethyl alcohol followed by air drying at room temperature, completely desorbed the dyes from the print and didnot damage the image material or the coating.

Another print, made as described, given the same treatment but in acetone, was completely decolorized without damage to the image material or coating.

Treatment of the photoconductive material of this example in benzene, xylene, toluene, carbon tetrachloride, and in Freon 113 did not desorb the dyes.

EXAMPLE 4 A sheet of photoconductive material as described in Example 1 but in which the zinc oxide was sensitized with EXAMPLE 5 A sheet of photoconductive material of the type described in Example 1 but sensitized with Fluorescein, a yellow merocyanine-type dye and Rose Bengal was exposed through an image to tungsten radiation. The image-wise volume conductivity pattern induced by this exposure was electrolytically developed to a visible image essentially of silver:manganese-iron hydroxide (or hydrated oxide) complex by the method described in Reithel U.S. Serial No. 45,947, filed July 28, 1960, now abandoned, but in which ferrous sulfate is used in place of the cobalt compound.

Samples of these developed prints were treated in each of the treatment baths of the following table. Following the treatment, the element was given a brief rinse in acetone and then air-dried at room temperature.

The concentrations of the organic acids used in this example are not to be considered as limiting since they can be varied considerably depending upon the binder(s),

' for 5 seconds.

dye(s) used in the photoconductive layer, the solvent or solvent mixture used, the treatment time desired, etc.

Table II Treatment Bath Treatment Photocon- Time in sec. ducting layer No. Composition 1 Acetone Completely g 1232 3 (about 2% by i 15 decolorized. 2 Acetone Benzoic acid (about 4% by 30 D0.

weight). 3..- Acetone Oxalic acid (about 4% by 30 Do.

weight 4 Acetone, 50 cc Acrylic acid (60% stabilized 30 Do.

with di-fl-naphthol), 3 cc. 5 Freon 113, 30 cc;

Ethyl alcohol, 20 cc 30 D0. Maleic acid, 1.5 g 6 Methyl isobutyl ketone, 30 cc 3 DO "as 7 et y et y etone, 8 Mpjleic acid,3b3 g l 5 pioxane, cc

Maleic acid, .5 g i 5 9 1,2-dichloroethane, 30 cc.

Ethyl alcohol, 30 cc" 20 D0. Maleic acid, 2 g 10.--. Tetrahydrofuran (stabilized with .1% hydroquinone), 30 Do. cc.

30 Do. Crotonic acid, 2.5 g l2 Acetone Fumaric acid (saturated 30 Do. 13 AsolutiomtS cetone, 5 cc Chloroacetic acid, 2 g. i 15 14.--- Acetone, 50 cc 30 D0 Glutaric acid, 2 g

Treatment bath 5 illustrates the use of ethyl alcohol as a solvent for maleic acid to make it possible to obtain the required maleic acid concentrations in the binder solvent Freon 113.

Another sheet of the same type of material bearing the silver-manganese iron hydroxide complex image material was immersed for 20 seconds in a bath containing 1 gram of 1,3-dichloro-5,S-dimethylhydantoin dissolved in 100 grams of the solvent mixture described in Example 1. This treatment completely desorbed the dyes from the layer and rendered them colorless without damaging the layer or image material.

Similarly, my process is used to advantage to decolorize the electrophotographic prints bearing other types of image materials. For example, a 2 percent solution of maleic acid in acetone completely removed the sensitizing dyes from a print made on the element described in Example 5, bearing a surface image of silver and/or silver sulfide electrolytically deposited from an aqueous solution of silver nitrate and thiourea.

The following example, using an aromatic hydrocarbon, benzene, as the binder solvent and acetic acid as the decolorizing agent will illustrate a method for determining the optimum decolorization formulation, duration of treatment and the after-treatment which depends not only on the rate of solution of the binder polymer(s) in the binder solvent, but on the rate of desorption of the dye(s).

EXAMPLE 6 A sheet of the photoconductive material described in Example 4 was not decolorized when bathed in benzene Longer treatment times, also, did not desorb the dyes and damaged the layer.

A series of S-second treatments was then made in baths of benzene containing increasing concentration of acetic acid with the concentration varying between 0.2 cc. and 8 cc. of acetic acid (glacial) per 50 cc. of benzene.

The treatments showed that complete decolorization, for the material, was obtained at glacial acetic acid concentrations above 9 percent, by weight. centration, decolorization was incomplete.

A sheet of this same photoconductive material, bearing an image material of the type described in Example 6 was completely decolorized when bathed in the benzeneacetic acid solution described above. The treatment did not damage the layer or image material.

The treatment process for removal of the sensitizing dyes from electrophotographic prints according to my invention is characterized from prior art methods by producing a complete desorption and/or decolorization of the dye(s) from the zinc oxide containing photoconductive layer. In most cases, the decolorized layers in the nonimage areas are as white or whiter (same or higher degree of neutral reflectivity) than similar undyed layers when coated on the same sub-strate. This is a valuable improvement over the prior art in which oftentimes the dyes are incompletely removed or the Zinc oxide is actually removed from the photoconductive layer, leaving a transparent underlayer having an image on its surface. The decolorization process of my invention is simple and readily adapted to fit in with either the Xerographic or photoconductographic process. Although the process requires an additional step, the decolorization process is fast and due to the high volatility of the solvents employed, the drying step is rapid.

The invention has been described in detail with particular reference to preferred embodiments thereof but it will be understood that variations and modifications can be eifected within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

I claim:

1. A process for decolorizing the sensitizing dye in the zinc oxide photoconductive layer of exposed and image developed electrophotographic elements comprising the step of contacting the said photoconductive layer with a solution of an organic acid selected from the class consisting of acrylic acid, crotonic acid, maleic acid, fumaric acid, acetic acid, chloroacetic acid, oxalic acid, glutaric acid and benzoic acid in an organic solvent selected from the class consisting of benzene, toulene, xylene, heptane, hexane, trichloromethane, trichlorofluoromethane, tetrachloromethane,- 1,1,2-trichloro-1,2,2-trifluoroethane, 1,2- dichloroethane, di-methylformamide, acetone, methyl ethyl ketone, methyl isobutyl ketone, methyl alcohol, ethyl alcohol, p-dioxane and tetrahydrofuran for a suflicient time to decolorize said layer, but an insuflicient time to dissolve any substantial quantity of said layer and an insufiicient time to damage said layer, and removing said solution from said layer.

2. The process of claim 1 in which the organic solvent is acetone and the organic acid is benzoic acid.

3. The process of claim 1 in which the organic solvent is a mixture of 1,1,2-trichloro- 1,2,2-trifluoroethane and ethyl alcohol and the organic acid is maleic acid.

4. The process of claim 1 in which the organic solvent is methyl ethyl ketone, and the organic acid is maleic acid.

5. The process of claim 1 in which the organic solvent is benzene and the organic acid is acetic acid.

6. The process of claim 1 in which the organic solvent is methyl isobutyl ketone, and the organic acid is maleic 'acid.

7. The process of claim 1 for decolorizing the sensitizing dye in the zinc oxide photoconductive layer of exposed and image developed electrophotographic elements in which the sensitizing dye is a triphenylmethane dye.

8. The process of claim 1 for discoloring the sensitizing dye in the zinc oxide photoconductive layer of exposed and Below this con- .irnage developed electrophotographic elements in which the sensitizing dye is merocyanine due.

(References on following page) 7 References Cited by the Examiner UNITED STATES PATENTS 10/ 1958 Sugarman 9671 10/ 1958 Greig 961 9/1962 Graig 96-1 11/1962 Schlesinger 96-1 4/1965 Sagura et a1 961 FOREIGN PATENTS 4/ 1956 Australia. 10/ 1959 Great Britain.

NORMAN G. TORCHIN, Primary Examiner.

' A. L. L1BERMAN,C. E. VAN HORN,

Asssitant Examiners.

@2 3? UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,250,611.], Dated J'LaY 10 1966 Inventor(s) Donald Eastman It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 6, line L B, delete the word, "toulene", and substitute in and line 70, delete the word,

its place --toluene--; "discolor'ing", and substitute in its place --decolorizing--.

oMiNtU Aim SEALED Maw (SEAL) Attest:

WILLIAM E. SCIHUYLER, JR.

Edward M. Fletcher, Ir.

Commissioner of Patents Attesting Officer 

1. A PROCESS FOR DECOLORIZING THE SENSITIZING DYE IN THE ZINC OXIDE PHOTOCONDUCTIVE LAYER OF EXPOSED AND IMAGE DEVELOPED ELECTROPHOTOGRAPHIC ELEMENTS COMPRISING THE STEP OF CONTACTING THE SAID PHOTOCONDUCTIVE LAYER WITH A SOLUTION OF AN ORGANIC ACID SELECTED FROM THE CLASS CONSISTING OF ACRYLIC ACID, CROTONIC ACID, MALEIC ACID, FUMARIC ACID, ACETIC ACID, CHLOROACETIC ACID, OXALIC ACID, GLUTARIC ACID AND BENZOIC ACID IN AN ORGANIC SOLVENT SELECTED FROM THE CLASS CONSISTING OF BENZENE, TOULENE, XYLENE, HEPTANE, HEXANE, TRICHLOROMETHANE, TRICHLOROFLUOROMETHANE, TETRACHLOROMETHANE,-1,1,2-TRICHLORO-1,2,2-TRIFLUOROETHANE, 1,2DICHLOROETHANE, DIMETHYLFORMAMIDE, ACETONE, METHYL ETHYL KETONE, METHYL ISOBUTYL KETONE, METHYL ALCOHOL, ETHYL ALCOHOL, P-DIOXANE AND TETRAHYDROFURAN FOR A SUFFICIENT TIME TO DECOLORIZE SAID LAYER, BUT AN INSUFFICIENT TIME TO DISSOLVE ANY SUBSTANTIAL QUANTITY OF SAID LAYER AND AN INSUFFICIENT TIME TO DAMAGE SAID LAYER, AND REMOVING SAID SOLUTION FROM SAID LAYER. 